1. Field of the Invention
The present invention relates to a method of metallizing an oxide ceramic with excellent hermetic sealing properties and brazing properties.
2. Description of the Related Art
The bonding of a metal to an oxide ceramic is widely utilized, for example, in the production of microwave output windows of magnetrons, vacuum switches, and the like, and for this purpose, the joint interface between the metal member and the ceramic member must have a hermetic sealing property. Accordingly, to ensure a good bonding, the ceramic member is generally metallized and then brazed to the metal member.
Most metallizing operations are carried out by the moly-manganese process, in which powders of refractory metals (metals having high melting points) such as molybdenum or wolfram are mixed with a manganese powder to form a powder mixture, which is then mixed with an organic vehicle to form a paste. The thus prepared paste is applied to an oxide ceramic, and the paste-applied ceramic is fired or calcined to form a molybdenum layer or a wolfram layer thereon. The ceramic member is then nickel-plated to form a nickel coating layer on the refractory metal layer and then brazed to a metal member. This nickel-plating is indispensable in the conventional moly-manganese process, since the wettability and the heat resistance of a brazing filler material become poor when the nickel coating is not formed, but the nickel-plating of a metallized surface formed by the moly-maganese process is very difficult and various treatments are necessary; i.e., a metallized surface produced by the moly-manganese process is partially coated with a glass phase, which must be removed, and an activation treatment with palladium chloride, an acid treatment and other treatments are necessary, to modify the surface property. As described above, the moly-manganese process is cumbersome in that it requires numerous processing steps which considerably increases the production cost, although it does provide an excellent hermetic sealing property.
The present inventors investigated a way in which the latter of the two steps of metallizing and nickel-plating could be omitted, since the metallizing step is essential but the nickel-plating step is merely assistive and not essential, and requires much equipment for the pretreatment, plating, and water treatment processes involved, for example. Note hereinafter the metallized layer composed of molybdenum or wolfram is referred to as the lower metallized layer and the layer containing Ni as a main component and deposited on the lower metallized layer is referred to as the upper Ni layer. A reduction of the steps needed for forming the upper Ni layer would lead to a considerable reduction of the total cost of the metallizing process, and from this point of view, the present inventors investigated ways in which the steps of forming the upper Ni layer could be completely omitted.
To omit the various steps of forming the upper Ni layer, it can be easily considered to utilize a two-step firing process in which a metallized layer is first formed on a ceramic by the moly-manganese process, a paste prepared by mixing Ni powder with an organic vehicle is applied to the first layer, and then a second firing is carried out. Although this process eliminates the need for plating equipment, nevertheless it requires two firing steps, i.e., the firing step must be repeated, and thus there is no reduction of cost. The present inventors proposed the simultaneous forming of the lower metallized layer and the upper Ni layer by a co-firing process in which a paste prepared by mixing a powder mixture of molybdenum and manganese with an organic vehicle is applied to a ceramic by screen printing, a nickel paste is applied by screen printing, and then firing is carried out. This process was not successful in practice because it was found that the co-firing caused a fusing of the Ni layer and reduced the strength of the lower refractory metal layer, probably due to the large difference of the melting point and the self diffusion coefficient of the two metals. Namely, it was proved that a sound metallized layer cannot be formed by a double coating of a refractory metal paste and a Ni paste. Although many double-coating and co-firing processes are known in the metallizing field, all of these processes relate only to an improvement of the formation of a lower metallized layer, and do not relate to a simultaneous formation of the lower metallized layer and the upper Ni layer by a single firing step, which is the object of the present invention. For example, Japanese Examined Patent Publication (Kokoku) No. 36-6542 discloses a process in which a first layer is formed of a mixture of a refractory metal and a ceramic, and a second layer is formed of a refractory metal. This process relates to an improved formation of the lower metallized layer, does not omit the steps for forming the upper Ni layer, and has a gist quite different from that of the present invention. The first and second layers are composed of the same metal, Mo, and the co-firing of the same metal is essentially the same as the firing of a single layer: Problems arise when two different metals such as W and Ni are to be fired simultaneously. Japanese Unexamined Patent Publication (Kokai) No. 58-213688 discloses a process in which an oxide powder of a high melting point metal is used as a first layer and a refractory metal is used as a second layer, to reduce the electric resistance of the metallized portion. Similar to Japanese Examined Patent Publication (Kokoku) No. 36-6542, this process relates to the formation of the lower metallized layer and has a gist different from that of the present invention, which covers the plating step. Japanese Unexamined Patent Publication (Kokai) No. 58-213688 does not provide a Ni layer and various steps for forming the upper Ni layer are necessary before brazing.
As mentioned above, to provide a good brazing property by metallizing an oxide ceramic, the surface of the ceramic must be coated with nickel and a high hermetic sealing property of the brazing is required when a bonded assembly is used as a material for electronic tubes. Conventional processes, however, have a drawback in that two steps of the formation of the lower metallized layer and the formation of the upper Ni layer are required, the latter including various treatment steps, which greatly increase the production costs.
Therefore, a method of metallizing which lowers the production cost while ensuring an excellent hermetic sealing and brazing properties is urgently required.